Melt spinning process



Oct. 16, 1951 J. E. WALTZ MELT SPINNING PROCESS Filed May 10, 1947 UMP,414 All HHHH a mu. M

A T TORNE'Y Patented Oct. 16, 1951 UNITED STATES PATENT OFFICE MELTSPINNING PROCESS Joseph Elmer Waltz, Wilmington, Del., assignor to E. I.du Pont de Nemours & Company, Wilmington, Del., a corporation ofDelaware Application May 10, 1947, Serial N 0. 747,252

18 Claims. 1

This invention relates to, the spinning of nylon yarn and moreparticularly to the production of yarn of uniform properties by spinningthe yarn by a process which controls the troublesome depolymerization ofthe polymer which occurs at spinning temperatures.

Commercial nylon is a synthetic linear condensation polyamide formed bythe condensation of suitable monomers or low molecular Weight polymerswith the elimination of water. Consequently, water is normally presentin the flake prior to spinning, the water being either a portion of thatformed during polymer production or an amount absorbed from thesurrounding atmosphere. It is well-known that at elevated temperaturesthe degree of depolymerization of polyamides is a function of the amountof Water present. To avoid excessive degradation by water steps areusually taken to dry the polymer flake to a low and controlled moisturecontent prior to the melt spinning step. In addition to using flakecontaining only small amounts of free moisture, steps may be taken tosweep away some of the water present during spinning by conductin thespinning in a moving atmosphere of very dry nitrogen or other suitablegas. By such means it is possible to obtain a satisfactory polymer.However, in the spinning of nylon yarn, a small inventory of polymermelt is maintained in the spinning cell for a short interval of timeprior to its passing to the metering pump and then to the spinneret. Theprocesses of polymerization and depolymerization can proceed during thismelted stage and, without control, yarn is produced having variablemolecular weight, and hence variable tenacities, elongations, dyeingcharacteristics and other non-uniform properties. While the polymer isstanding in the melt stage it is virtually impossible to remove all themoisture and to keep operating conditions constant. Thus, the amount andkind of nitrogen used, the unit temperatures maintained, the gaspressures and similar conditions vary considerably. Because of thecritical nature of these known processes it is extremel diflicult toproduce a yarn having optimum uniformity of physical properties.

Accordingly, it is an object of this invention to provide a processwhich avoids the difficulties referred to above and which results in yarof high quality and uniform properties. A further object is theprovision of means for controlling the polymerization-depolymerizationequilibrium. A still further object is the provision of a process bywhich yarn having very good uniformity may be obtained without the useof costly materials such as highly purified nitrogen or a simiv lar gas.Other objects will appear hereinafter.

These objects are accomplished by the following invention in whichspinning is conducted under a positive and predetermined water vaporpressure. The nylon polymer flake is melted in the presence of watervapor, the water vapor pressure being controlled. The melt is then spunwithout subjecting it totemperature variations or changes in water vaporpressures. In practice, the polymer flake is melted in the presence ofsteam the pressure of which is controlled. The melt is allowed to standfor a brief period until equilibrium is reached and it is then meteredand spun under conditions which keep the effect of the steam constant.

This invention will be more clearly understood .by reference to theattached Figure 1 which is a diagrammatic, side elevation view of asuitable melt spinner.

In Figure 1, in which like numbers refer to like parts, the polymer isfed into the melt spinner through the polymer flake supply duct In. Theflake passes down through the open hopper I I into a preheater [2. Thispreheater is heated in any convenient way, as, for example, by means ofa steam jacket l3. Steam is passed into the heated jacket at I [and ispassed to the trap at the bottom of the heater at point l5. At point I 6a portion of the steam is removed and is sent through the bypass I! inwhich is located a valve l8. The flake after being preheated passesdownward to point l9 at which'it comes into contact with the steampassing through valve l8. The steam delivered to the preheater jacket !3is preferably high pressure steam (10- pounds per square inch) andthereby provides a temperature of the order of to C. The steam thatpassed through valve I8 is at atmospheric pressure at point 19 since thehopper l Lis open to the atmosphere. Operation in this manner providesan automatic control of steam pressure and it is possible to keep thepressure in the melt spinner within very narrow limits. The polymer meltis in equilibrium with steam and, accordingly, the viscosity of the meltis uniformly controlled. The steam introduced into the flake at point l9passes up through the polymer flake and out into the air at point 20.Thus, the steam serves to blanket the polymer and to adjust .theviscosity of the melt in relation to the water vapor pressure of thesteam.

After mixing with the steam, the flake passes to a grid melter 2| whichmay be heated by steam or electricity or other means. The polymerbecomes thoroughly melted and forms a melt pool 22. Polymer is thenmetered out through the pump 23 which passes the melt in fixed andconstant amount to the spinneret assembly 24. The inventory of polymerin the melt 22 is kept as small as possible in order to preventdecomposition. It is, however, preferred to keep an amount sufficientlylarge enough so that the polymer after melting stands exposed to thesteam for a minimum of about 15 minutes before being metered out. Thisexposure permits establishment of an equilibrium.

In the conventional nitrogen processes the amount of water in the meltranges from 0.11% to 0.18% even under the most rigid control. Further,the water vapor pressure varies from 550 mm. to 850 mm. of Hg. Thisvariation results in yarn non-uniformities as mentioned above. By theprocess of this invention it has been found possible to maintain aconstant amount (0.16% for polyheXamethylene adipainide) of moisture inthe melt, and a more constant pressure (710- 7'70 mm. of Hg) in thecell. This constancy in moisture content results in an accuratelycontrolled polymerization-depolymerization equilibrium and the spunpolymer has physical properties which are much more uniform thancomparable polymer spun by the conventional nitrogen processes.

The amount of water in the melt will depend, among other 'factorsup'onthe polyamide being spun. Thus, while the amount is 0.16% forpolyhexamethylene adipamide, the figure for other during spinning anddoes not vary between certain limits prevailing when the particularpolyami'de is melt spun by conventional processes.

Similarly, the pressure in the spinning unit will depend, in part, uponits location or; in other words, the local atmospheric. conditions. Ingeneral, the maximum fluctuations will be :30 mm. of Hg. The main pointis that the pressure in the spinning unit during spinning by the processof this invention will be very nearly equal to the atmospheric pressure.Since the atmospheric pressure at a given location generally fluctuatesonly :30 mm. of Hg, the pressures in the spinning cells will besubstantially constant. As pointed out above, by the process of thisinvention the fluctuation in pressure within the cell is advantageouslymaintained within narrow limits.

The advantages obtainable by use of the process of this invention may beseen by comparing the physical properties of yarn produced by theprocess of this invention to the physical properties of yarn produced byconventional spinning methods using purified nitrogen as the atmosphereover the melt pool. The examples which follow are given for illustrativepurposes and are not to be considered as limitative. In the standardprocess using polyhexamethylene adipamide and anitrogen pressure of 20pounds per square inch, a yarn was produced having an average tenacityof 4.59 grams per denier with 'an overall standard deviation of 0.234.When the same polymer was spun by the process of this invention theyarns had a comparable average tenacity (4.43 grams per denier) but theoverall standard deviation was considerably lower, being only 0.148. V

Similarly, polyhexamethylene adipamide after spinning showed for theyarn spun by the process of this invention a relative viscosity of 26.0(8.4% solution of polymer in formic acid) with an overall standarddeviation of only 0.53. Comparable polymer spun by the conventionalspinning methods had a similar average relative viscosity (27.7) but amuch higher overall standard deviation of 1.04. This shows that thepolymer spun by the process of this invention had a more uniformmolecular weight than the polymer resulting after spinning by thestandard spinning method using nitrogen as the atmosphere over the meltpool.

In addition to greater uniformity in'molecular weight and in tenacities,the yarns of this invention had greater uniformity in elongations. Theaverage elongation figures for the yarn spun by the process of thisinvention was 24.0% as compared to an average elongation of 22.1% foryarn spun is a similar manner by the conventional process. The averagestandard deviations for the two yarns, however, were 2.28 and 2.42,respectively, showing the greater uniformity in this respect of theyarns of this invention.

These advantages in uniformity disclosed are summarized in the followingtable:

TABLE I Yarn Property Control 1.04 g Spun" }.\Iolccular Weight. 24ontrol. 0.2 e tea n Spun }Tenamty o. :48 0'1rol 2. 2 Steam SpunJmongatmn 2.28

TABLE II Dye junctions in IOU-yards of woven fabric Number of JunctionsDepth Control Steam-Spun 1 Summation of the product of the number ofjunctions per 100 yards of fabric and the depth of each.

From the above table it can be seen that the acid-dye variations for theyarn of this invention are only one-fourth as great as in control yarn.In other words the dye coverage or steam spun yarn with acid-dye isabout four times better than the coverage in the acid-dyed controlledfabric. Prior to this invention the variation in physical and chemicalproperties of the yarn .pre-

cluded the commercial use of acid-dyes and it was necessary to rely uponthe use of acetate dyes. The process of this invention produces a yarnhaving physical and chemical properties the uniformity of which permitsthe use of acid-dyes, the dyed products being equivalent in uniformityto commercial nitrogen-spun yarns dyed with acetate dyes. The commercialuse of acid-dyes is desirable since a greater variety of dyed productsmay be produced through the use of this additional class of dye-stuffs.

While the yarns in the above examples were prepared using dry steam atatmospheric pressures, steam at higher pressure may be used. The steampressure may be as high as 30 pounds per square inch gauge but generallyit is preferred to use steam pressure within the limits of 7 to 10pounds per square inch gauge. The maximum steam pressure which can beused is dependent upon factors such as the nature of the polyamide beingspun, and the type of yarn being formed. In general, the higher thesteam pressure the higher is the amount of water in the melt inequilibrium with the steam. The

higher the pressure of the water vapor the lower Thus, it is preferredto use dry steam at atmospheric pressure because the process is muchsimpler. High pressure apparatus is not needed.

Further, the steam when used at atmospheric pressure does not tend tokeep steam and gases of decomposition dissolved in the polymer to theextent that steam under pressure does. Further, a free flowing,controlled volume of steam passing over the melted polymer and throughthe flakes in the preheater provides a satisfactory means foreliminating occluded air.

While the process of this invention has been 1 described in particularreference to polyhexamethylene adipamide, many other synthetic linearpolyamides may be used. For example, the

following may be mentioned: polytetramethylene-adipamide,polytetramethylenesebacamide. polypentamethyleneadipamide,polypentamethylenesebacamide, polyhexamethylene adipamide,polyhexamethylenesebacamide, polyhexamethyleneglutaramide,polyoctamethyleneadipamide, poly p xylylenesebacamide, and the polyamidederived from 3,3 diaminodipropyl ether and adipic acid. Polymerizedepsilon-amino-caproic acid, polymerized Q-aminononanoic acid, andpolymerized ll-amino-undecanoic acid are examples of linear polymers,derived from amino acids, which may also be used. Further, copolyamidesor interpolyamides and any mixtures of polyamides :may be spun, by theprocess of this invention to form yarns of high quality. In general anypolyamide, which includes the polyesteramides, may be used, such as aredescribed in U. S. 2,071,250, U. S. 2,071,253, U. S. 2,130,523, U. S.2,130,948 and U. S. 2,190,770. While a specific melt spinner has beendescribed this invention is not limited thereby and any equivalent orsimilar device may be used.

By the process of this invention it has been found possible to usepolymer flake of moisture content as high as 6.5%. Contrary to thepresent practice of the industry, it is not required in the working ofthis invention to use polymer flake of low and controlled moisturecontent. This is a matter of considerable importance since the processbecomes more economical. Generally, it is preferred to use flake whichhas reached equilibrium with ordinary atmospheric conditions. Such flakecontains about 4% moisture. No great or costly effort is necessary tocondition the flake prior to spinning such as, for example, conditioningin atmospheric pressure of controlled humidity. Generally, mere exposingflake to normal atmospheric conditions is adequate. This is a markedadvantage since no special equipment is required.

The results of the process of this invention are surprising and contraryto the literature and expectation of one skilled in the art. Sincemoisture plays an important role in the degradation of nylon held atelevated temperatures, it would appear unlikely that yarn of uniform andsatisfactory properties could be obtained by melt spinning the polymerunder a positive Water vapor pressure and in the presence ofconsiderable amounts of water vapor. However, as shown above, it hasbeen found that this can be done by the process of this invention whichprovides yarn of the desirable quality in a surprisingly simple manner.Yarn having very uniform properties in respect to tenacity andelongation and dye depths and having high and uniform molecular weightsis produced by the process of this invention. The necessity of usingequipment for operating under a positive pressure of nitrogen gas poundsper square inch) is eliminated. Thus, the process is freed of manyproblems of equipment, design construction and operation. It is alsofreed from the required maintenance of feeding locks and storage locksand the repeated purging of them in order to remove air. The operationof this invention does not require care fully predried flake nor largevolumes of purified gas as an inert atmosphere. The criticalness whichthe standard methods for spinning under nitrogen required is eliminatedby the process of this invention. It is much easier to control the watervapor pressure in the process of this invention which involves the useof only one gaseous material (steam) than in the conventional processeswhich involve the use of two gases (water vapor and nitrogen).Surprisingly and advantageously, yarn of the greater uniformity isproduced in a practicable manner and with a. reasonable requirement ofcriticalness.

Any departure from the above description which conforms to the presentinvention is intended to be included claims.

I claim:

1.- A process for spinning a synthetic linear polyamide which comprisesmelting the said polyamide in a chamber; introducing water vapor intosaid chamber at a substantially atmospheric pressure; exposing theresultant melted polyamide to the said water vapor until water isuniformly distributed in the melt; and feeding the resultant moltencomposition to a means for extruding it from said chamber, the meltingand the feeding being carried out at substantially atmospheric pressureand in the presence of Water vapor at substantially constant pressure.

2. A process for spinning a synthetic linear polyamide which comprisesmelting the said polyamide in a chamber; introducing water vapor intosaid chamber at substantially atmospheric within the scope of thepressure; exposing the resultant melted polyamide to the said watervapor until water is uniformly distributed in the melt; and feeding theresultant molten composition to a means for extruding it from saidchamber, the melting and feeding being carried out at substantiallyatmospheric pressure at a substantially constant vapor pressure of waterand the said molten composition having a substantially constant moisturecontent prior to extrusion.

3. In a process for melt spinning a synthetic,

linear polyamide the steps comprising melting said polyamide in achamber; introducing into said chamber water vapor at substantiallyatmospheric pressure; exposing the resultant melted polyamide to thesaid water vapor until Water is uniformly distributed in the melt; andfeeding the said polyamide to a means for extruding it from the saidchamber, said melting and feeding being carried out at substantiallyatmospheric pressure and in the presence of a substantially constantvapor pressure of water.

4. In a process for melt spinning a synthetic linear polyamide, stepscomprising melting said polyamide in the presence of a Water vapor atsubstantially atmospheric pressure, said vapor being introduced intosaid chamber; exposing the resultant molten composition to said vaporuntil water is uniformly distributed in the said molten composition; andfeeding the said molten composition to a means for extruding it fromsaid chamber, the said steps being carried out at substantiallyatmospheric pressure.

5. A process in accordance with claim 1 wherein the said polyamide ispolyhexamethyleneadipamide.

6. A process in accordance with claim 1 wherein said polyamide ispolyhexamethylenesebacamide.

7. A process in accordance with claim 1 wherein said polyamide ispolymerized epsilon-aminocaproic acid.

8. A process for spinning a synthetic linear polyamide which comprisesmelting said polyamide in a chamber; introducing water vapor atsubstantially atmospheric pressure chamber; exposing said polyamide tosaid vapor which isin the form of steam substantially at atmosphericpressure; allowing the resultant .molten composition to come intoequilibrium with said water vapor so that water is uniformly distributedin said molten composition; and feeding said molten composition atsubstantially atmospheric pressure and in the presence of said watervapor to a means for extruding it from into said of said vapor to ameans for extruding it from said chamber.

10. A process for maintaining the rate of polymerization and rate ofdepolymerization at equilibrium in a molten synthetic linear polyamidewhich comprises heating, at substantially atmospheric pressure, saidpolyamide in an atmosphere consisting of water vapor at substantiallyatmospheric pressure until water is uniformly distributed in saidpolyamide.

11. A process in accordance with claim 10 in which said pressure is fromabout I10 to about 779 mm. of mercury.

12. A process in accordance with claim 10 wherein said polyamide ispolyhexamethyleneadipamide.

13. A process in accordance with claim 10 wherein said polyamide ispolyhexamethylenesebacamide.

l4. A process in accordance with claim 10 wherein said polyamide ispolymerized epsilonamino-caproic acid.

15. A process for spinning a synthetic linear polyamide which comprisesmelting the said polyamide in a chamber the atmosphere of which consistsof water vapor; introducing the water vapor into said chamber at asubstantially constant pressure of up to 10 pounds per square inchgauge; exposing the molten polyamide to said water vapor until water isuniformly distributed throughout the molten polyamide; and feeding theresultant molten composition to a means for extruding it from saidchamber, the said melting and the said feeding being carried out in thesaid atmosphere consisting of said water vapor.

16. A process for keeping the rate of polymerization occurring in amolten synthetic linear polyamide equal to the rate of depolymerizationoccurring in said polyamide which comprises heating said polyamide in anatmosphere consisting of Water vapor at a pressure of up to 10 poundsper square inch gauge until water is uniformly distributed throughoutsaid polyamide.

17. A process in accordance with claim 16 in which said heating iscarried out for at least 15 minutes.

18. A process in accordance with claim 1 in which the polyamide ispolyhexamethylene adipamide and the amount of water uniformlydistributed therein is about 0.16%.

JOSEPH ELMER WALTZ.

REFEBENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Num er Name Date 2,253,176 Graves Aug. 19, 19412,295,942 Fields Sept. 15, 1942 2,303,340 Dreyfus Dec. 1, 1942 OTHERREFERENCES Maxness: Preheating with Steam, Plastics, February 1947,pages 34-36, 39, 90.

